Voltage blocking, for example, is known as a process for applying a high voltage to an electrically conductive coating material to apply an electrostatic coating to a workpiece such as an automobile body or the like. According to this process, the electrically conductive coating material is temporarily introduced into an intermediate storage reservoir (intermediate storage mechanism) which is insulated from the ground potential, and thereafter a supply passage interconnecting the intermediate storage reservoir and a coating material supply source is washed and dried to form a voltage block. Then, the electrically conductive coating material to which a high voltage is applied is supplied from the intermediate storage reservoir to a coating gun, which applies an electrostatic coating to the workpiece.
One electrostatic coating apparatus for use in the above coating process is known from Japanese Laid-Open Patent Publication No. 6-60452, for example. According to Japanese Laid-Open Patent Publication No. 6-60452, as shown in FIG. 11, a pump 1 is provided as an intermediate storage mechanism, and has a coating material inlet port 1a supplied with an electrically conductive coating material from a coating material supply passage 2. The pump 1 delivers under pressure a predetermined amount of electrically conductive coating material from a coating material outlet port 1b to a coating machine 3.
The pump 1 has a piston 4 movable back and forth by high-pressure air that is supplied from an air supply source 5 through a pressure regulating valve 6. A piston rod 4a coupled to the piston 4 moves at a speed that is detected by a non-contact sensor 7. Based on the moving speed of the piston rod 4a, the flow rate of the electrically conductive coating material supplied to the coating machine 3 is measured.
The measured flow rate of the electrically conductive coating material and a flow rate that is preset dependent on the amount of coating material ejected from the coating machine 3 are compared with each other, and the pressure of the high-pressure air supplied to the pump 1 is variably adjusted by the pressure regulating valve 6 depending on the difference between the compared flow rates. The pump 1 can be reduced in size, and the amount of electrically conductive coating material that is stored in the pump 1 can be made constant.
For supplying the electrically conductive coating material supplied to the pump 1 to the coating machine 3 for the coating process, a process for filling the pump 1 with the coating material is performed, and air tends to be mixed with the coating material when the pump 1 is filled with the coating material. At this time, air which is mixed with the coating material and introduced into the pump 1 is liable to move upwardly in the pump 1.
On the pump 1, the coating material outlet port 1b is positioned upwardly of the coating material inlet port 1a. Therefore, as shown in FIG. 12, when the piston 4 moves forward (in the direction indicated by the arrow X) to supply the coating material from the pump 1 to the coating machine 3, air trapped in an upper portion of the pump 1 is delivered from the coating material outlet port 1b to the coating machine 3. Therefore, the coating material ejected from the coating machine 3 to the workpiece contains air mixed therewith and fails to form an appropriate coating pattern, so that a highly accurate electrostatic coating process cannot be performed.
For changing coating material colors to use a new coating material having a different color, the interior of the pump 1 is washed. At this time, a washing liquid introduced from the coating material inlet port 1a into the pump 1 is discharged from the coating material outlet port 1b. However, since the coating material outlet port 1b is positioned upwardly of the coating material inlet port 1a, the washing liquid tends to remain trapped in the pump 1, and the new coating material that is introduced into the pump 1 after it has been washed is mixed with the remaining washing liquid. Consequently, the amount of coating material to be discarded is increased, making the electrostatic coating apparatus uneconomical, and there is a danger of performing an electrostatic coating process using the coating material mixed with the washing liquid.
According to Japanese Laid-Open Patent Publication No. 6-60452, furthermore, the electrostatic coating apparatus is often mounted on a robot for automatically performing the electrostatic coating process. It is desirable that the electrostatic coating apparatus as a whole be made compact and mounted on a robot. It is thus necessary that an insulating mechanism 3 be disposed closely to the pump 1.
To prevent the electrically conductive coating material from leaking, the pump 1 is made of an insulative resin material. However, because a high-voltage generating means is incorporated for applying a high voltage to the electrically conductive coating material, the high voltage tends to leak along the surface of the pump 1 to the insulating mechanism 3, causing a dielectric breakdown.
Heretofore, the pump 1 and the insulating mechanism 3 need to be spaced a relatively large distance from each other, with the result that the electrostatic coating apparatus cannot be made compact as a whole.
With the electrostatic coating apparatus of the above type, as shown in FIG. 11, an insulating section 8 is provided between the pump 1 and a coating material supply 2a. The insulating section 8 has valve mechanisms 8a, 8b and an insulating pipe 2b connected between the valve mechanisms 8a, 8b and serving as the supply passage 2. A damping path D1 can be connected to the valve mechanism 8a, and the coating material supply 2a, a washing unit 9, and damping path D2 can selectively be connected to the valve mechanism 8b. 
For applying an electrically conductive coating material of the same color with the above electrostatic coating apparatus, the coating material supply 2a is connected to the supply passage 2 through the valve mechanisms 8a, 8b, and fills the pump 1 with the electrically conductive coating material through the supply passage 2. Then, the valve mechanism 8a is actuated to connect the insulating pipe 2b to the damping path D1, and the valve mechanism 8b is actuated to connect the washing unit 9 to the insulating pipe 2b. 
The washing unit 9 supplies a washing liquid to wash the insulating pipe 2b between the valve mechanisms 8a, 8b. Thereafter, the washing unit 9 supplies drying air. The interior of the insulating pipe 2b is now washed and dried, electrically insulating the coating material supply 2a and the pump 1 from each other. The pump 1 is now actuated to supply the electrically conductive coating material from the pump 1 to the coating machine 3, and a high voltage is applied to the electrically conductive coating material to apply an electrostatic coating to the workpiece (not shown).
As described above, each time the pump 1 is actuated to deliver the electrically conductive coating material under pressure to the coating machine 3 to apply an electrostatic coating, the interior of the insulating pipe 2b is washed in the insulating section 8. At this time, the electrically conductive coating material which remains in the insulating pipe 2b is drained from the insulating pipe 2b into the damping path D1 each time the insulating pipe 2b is washed.
Accordingly, the electrically conductive coating material which is unused in the insulating pipe 2b is unnecessarily drained in each washing process, resulting in an increase in the used amount of electrically conductive coating material. Particularly, if the electrostatic coating process is carried out for a long period of time, then the amount of electrically conductive coating material that is drained from the insulating pipe 2b is considerably increased, making the electrostatic coating apparatus highly uneconomical.